By day, Franciscus Donders was a mild-mannered opthamologist. By night, he decided to figure out how quickly thoughts travel in people's brains. By shocking their feet.

Like light, thought travels so fast that it takes most civilizations centuries to figure out that it travels at all. People knew that it took time to gather thoughts together, or to come up with a complicated idea, but the idea of thought being an impulse that traveled through the body was something entirely different.

And even when people realized how your nerves worked, the task of timing the processes of your brain was daunting. Unlike light, thought only moves in a limited amount of space. You can set a lamp down a few miles away, and figure out how long it takes for light to travel. Thought, meanwhile, carries no more than about six feet. It was hundred of years before people made clocks capable of timing such a fast impulse over such a small area.

In 1865, though, a clockmaker called Matthäus Hipp sent to Utrecht University a chronometer capable of timing one five-hundredth of a second. This gave an opthamologist and physiologist, working at the university, an idea. Franciscus Donders wanted to know how fast different mental methods took and now he had a way to time them. He began by using tests that anyone today would be happy to take. He ended by getting a little more extreme.

Donders was most interested in the difference between automatic response and reasoned response. He wanted to know the difference that actual thought made. To measure that difference, his experiments gradually got more complex. He'd start by asking people to press a buzzer when they saw a lamp, and then press a buzzer only when they saw a green lamp. He'd ask them to signal when they heard the letter "v" from a phonograph, and then buried "v" in the middle of other letters to see how long it took people to distinguish it. But just to make sure he was right? He'd shock people's feet.

His most well-known experiment was one in which his subjects — who hopefully had been well compensated — placed their feet against pads hooked up to electricity. Sometimes Donders would tell them to press a telegraph key when their feet were shocked. Sometimes he'd tell them to press the key only when their left foot was shocked. And sometimes he'd tell them to press one key when their left foot was shocked and another when their right foot was shocked. Each of these variations had different reaction times. Donders attributed this to the mental effort involved in each task. Simple reactions, like pressing a key when anything at all happened, required almost no mental effort, and so came fast. Discrimination reactions, which required the subject to filter out some of the stimuli and only react to one, took a little longer. And choice reactions, when people had to register what happened each time and respond differently to each stimulus, took the most time of all.

Although Donder's couldn't track the path a nerve impulse took through a person's brain and back to their body, his process did help him estimate the speed of thought. The current reaction time required for athletes at Olympic events, the time before which they are considered to have made a false start, is 0.1 seconds. That is comparable to Donder's simple reaction results. Looks like the old foot-shocker did a good job.